I would really appreciate any advice on dialing in the timing in my truck.

new 390 bored 0.03" and pretty stock otherwise
Edelbrock Performer Aluminum Intake
Edelbrock 1406 Carb (4bbl)
Pertronix Flame Thrower distributor and coil (using the medium springs and 16deg -about- limited mechanical advance tabs)

I got out my new timing light and graphed my advance.

-Warmed up and idling at 650 RPM, my initial timing is 6 deg. BTDC
-Mechanical advance maximizes at 2000 RPM with 14 deg additional for a total of 20 deg BTDC.
-I plugged in the vacuum advance and it maximized between 2000 and 2500 RPM with an additional 20 deg. for a total (including initial) of 40 deg BTDC.

I have not driven it much yet. It starts up nice and easy. I drove up a long and very steep residential hill today and there was no pinging/knocking or loss of power. On flat ground I opened the secondaries a little while in 3rd gear (probably 45-50 MPH) and it had some nice power.

I have lighter and heavier springs for the distributor, and options to limit the mechanical advance at either 12 or 20 deg instead of the current 14.

Can anybody help me interpret this data?

Thanks everyone for all the help along the way. It's fun having this thing running again.

 

Typical V8 numbers are maybe 10 to 12 initial and around 24 mechanical. "Total" timing is then usually 34 to 36 degrees BTDC "all in" before 3000 RPM. Finally, reconnect vacuum advance and back off till no ping at steady cruise or part throttle acceleration. This will result in around 50 degrees of timing at cruise highway speeds, lightly loaded.

 

Advance your initial timing to about 10 and you'll be surprised at the pickup it gains.

Not to mention cooler/smoother idle, better fuel economy, snappier starting.

It's a win-win-win!

 

Is there a maximum total advance I want to avoid? If the engine doesn't ping or loose power, does that mean things are ok?

 

Yes, too much timing will break rings or burn a hole in a piston.

But the idea is, any engine will run best with the maximum possible ignition advance at all times, under all conditions, just short of any ping or predetonation.

Modern engines use knock sensors coupled with computerized ignitions to achieve this; distributors use mechanical centrifugal weights and springs, based on engine RPM, this is coupled with an engine load dependent vacuum diaphragm connected to the movable breaker plate to advance and retard engine timing, these two systems work completely isolated of each other.

Disconnect and plug vacuum advance, and run it up to see what the distributor mechanical advance + initial ends up at. The initial timing isn't so important, adjust it wherever it needs to be so that you end up with around 34 or 36 degrees total.

Usually 8 or 10 degrees initial, maybe a little more, is where a V8 likes to be. You may need to limit the mechanical advance some to keep the total within 34 or 36. Usually the stock OEM advance curve is too conservative RPM wise, installing lighter springs allows the mechanical advance to come in at earlier RPMs, this makes a big difference. Dialing it in will achieve the best performance and fuel economy, run cooler, less pollution etc.

http://www.gofastforless.com/ignition/advance.htm

 

That's what I did!

" -Warmed up and idling at 650 RPM, my initial timing is 6 deg. BTDC
-Mechanical advance maximizes at 2000 RPM with 14 deg additional for a total of 20 deg BTDC.
-I plugged in the vacuum advance and it maximized between 2000 and 2500 RPM with an additional 20 deg. for a total (including initial) of 40 deg BTDC. "

" I have lighter and heavier springs for the distributor, and options to limit the mechanical advance at either 12 or 20 deg instead of the current 14. "

So... In order to achieve 38 deg at around 2500 RPM without vacuum, I would need to switch my mechanical advance limiter inside the distributor to the 20 deg option, then set my initial to 18. Once my vacuum is hooked back up, it adds another 20 deg.

Is this what you mean?

 

Distributor degrees are normally measured at half crank degrees. So read the Pertronix instructions carefully on this. Typically about 20 to 24 or so (crank) degrees is a hot setup. Then dial in your crank timing to get 34 to 36. The only thing that matters is what shoes up on your damper timing marks I suppose.

It would also be unusual though, to be "all in" by 2000 RPM. That's too hot, I'm pretty sure! You need to check carefully that the mechanical advance is for sure done doing its thing, past 3000, and on up. Maybe "all in" by 2800 or so will work for you.

Vacuum advance is not counted for "total" timing oddly enough, just crankshaft + mechanical. Disconnect and plug vacuum advance when checking for the 34 to 36. Do you have the damper marked out to 40 or 50 degrees? Flourescent timing tape makes this easier to read. Stay out of the way of the plane of the cooling fan, in case it decides to let go while spooling up the RPMs.

You can measure the vacuum advance on the damper but there is no load on the engine in neutral, so it probably won't really be 20 degrees vacuum advance in operation. It will be a lot though, over 10. Keep the vacuum advance plugged and disconnected till you are satisfied with how it runs with only the mechanical in use.

Then reconnect vac, start out high and back it off till there's no ping or only very light ping ocassionally on part throttle acceleration. You should (for now anyway) be connected to a source of ported vacuum for the vacuum advance canister.

 

Okay I read some of the instructions provided with the Pertronix distributors. I think they are listing for crank degrees, this is not how OEM distributors are measured. Normally distributor degrees are half of crankshaft degrees, so a "13L" reluctor slot for example, on an OEM distributor corresponds to an actual 26 degrees timing measured on the crankshaft. The Pertronix instructions makes no mention of this.

Set the 20 degree mechanical up, and try 12 to 14 degrees of initial timing, in any case you want to see about 34 degrees say by 3000 RPM, vacuum advance disconnected. Most V8 run and idle better at around 10 degrees initial (or more), this is why the mechanical advance has to be limited with stock OEM distributors, they have way too much mechanical advance from the factory, and it also comes in too late (often above 4000 RPM) You should use the silver springs to start.

Too much initial advance though can cause kickback of the starter, especially when engine is hot.

 

Pertronix is in fact listing crankshaft degrees. And, they seem accurate. I'm using a timing light that lets you zero out and measure advance. I measured 14 degrees of mechanical advance while using the "blue" limiters rated at 16 degrees.

So it seems i may want to:
bump my initial up to the 10 -14 range
switch the limiters out for the 20 degree ones,
add stiffer springs because I'm all in at 2000 rpm

Oh, my vacuum advance is not adjustable, so if it starts pinging with the above setup, I'll have to retard my timing until the pinging goes away?

 

Sounds like you're on track. I see that listed in some of the instructions provided, listed as crankshaft degrees.

And a non-adjustable vacuum canister. Hm. See how it works first with your proposed curve. Remember ping from a dead stop indicates crank timing, excessive vacuum advance is a different deal, so don't back off on the initial timing simply because it's pinging on the high side. This will work, but it's not really the way to go about it. Remember vac advance is only load dependent and comes into play at steady speeds or at part throttle when engine vacuum comes back up. Listen carefully at say 50 or 60 steady cruise on the highway. Excessive vaccum advance here, the engine will sound a little "ragged" or surge and even maybe a constant ping. A little ping every now and then upon part throttle acceleration is OK. Check your plugs after a highway run, they will show effects of engine timing.

Since vac advance is not adjustable you're kinda stuck. I've read some folks will swap it out with adjustable cans. But if you get it dialed into the 34 or 36, possibly a little more total timing range, I suppose it should be just right. Pertronix knows their stuff.

 

I run my initial at 20* and my mechanical is about 16-17* for a total of 36-37* and all in by 2800. To set the initial I hooked up a vacuum guages and adjusted timing until I got the best reading and idle. Came out at 20*. most fe's will like about 36-40* total.

 

Is that with pump gas, or premium? Some suggest 34 to 36 for higher compression, mild cam engines, 36 to 38 for more stock engines. No starter kickback with 20 initial? The more advance you can get, at idle it will run cooler.

It is good to experiment and find out what's what, each engine is a little different.

 

I know this is apples to oranges but I run 18 Initial on my Cleveland, with 24 mechanical all in at 3500.

Good lord that's 42 degrees. Maybe I should back it down a little haha.

 

That's with 87 oct. I'm running flat tops with 4 valve relief and compression is right around 10.1. Starts great at 20* and no kickback. It really made the truck feel better with my timing set up this way.

 

I run 87 too. Same way you do, I turned it up until it felt good. I do think I should back it off at least a little bit though. I'll get spark knock when I shut it down sometimes.